Community automatic exchange network



14 Sheets-Sheet l Original Filed July 22. 1947 oos ATTYS.

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swITCH BOARD TOLL OFFICE |0.. INVENTOR. 2 BY PIer Bakker 22%,wfwgdwni AHyIs.

Dec. 25, 1951 BAKKER 2,579,721

COMMUNITY AUTOMATIC EXCHANGE NETWORK Original Fild July 22, 1947 C327 A.T.B. TONE 14 Sheets-Sheet 5 Dec. 25, 1951 BAKKER 4 2,579,721

COMMUNITY AUTOMATIC vEXCHANGE NETWORK Original Filed July 22, 194'? 14 Sheets-Sheet 4 salu- W ",.4l5 A\\446 46p n 648| TONE C48 TOLL LINE CIR. 250 R440 R4|o i LMEP sElzE MOL 4wd 48o E E REV "Pol R445 STMRT E I' A 45| 452x? MOT. st c4|3 448 vr Aux TN EST."C'4T4 TANDEM OFFICE 3o E C481 48M @F-L @L C4714 470\ l c4723 c473\A 49v0\\ 4 INVENTOR.

Pier Bakker Dec.

Original Filed July 22, 1947 P. BAKKER COMMUNITY AUTOMATIC EXCHANGE NETWORK 14 Sheets-Sheet 5 Dec. 25, 1951 COMMUNITY AUTOMATIC EXCHANGE NETWORK Original Filed July 22. 1947 P. BAKKER V3.4 Sheets-Sheet 6 JNI/"ENTOR Pier Bakker h l M,

Atty s.

De- 25, 1951 P. BAKKER 2,579,721

COMMUNITY AUTOMATIC EXCHANGE NETWORK Original Filed July 22. 1947 14 Sheets-Sheet 7 LIN ANS.

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. Pier Bakker BY I M' MMG/@wmf Dec. 25, 1951 COMMUNITY Original Filed July 22, 194'/ i CSOI: SIBE P. BAKKER 2,579,721

AUTOMATIC EXCHANGE NETWORK 14 sheets-sheet a 25, 195} p, BAKKER 2,579,721

COMMUNITY AUTOMATIC EXCHANGE NETWORK I Original Filed July 22, 1947 14 Sheets-Sheet 9 Dec. 25, 1951 i COMMUNITY AUTOMATIC EXCHANGE NETWORK Original Filed July 22, 1947 P. BAKKER 2,579,721

14 Sheets-Sheet lO Dec. 25, 1951 p, BAKKER 2,579,721

COMMUNITY AUTOMATIC EXCHANGE NETWORK c Vus? INVENTOR. Pier Bakker BY l MIMMM Dec. 25, 1951 P. BAKKER 2,579,721

COMMUNITY AUTOMATIC EXCHNGE NETWORK Pier Bakker BY A WT/AMM Dec. 25, 1951 p, BAKKER 2,579,721

COMMUNITY :AUTOMATIC EXCHANGE NETWORK- Original Filed July 22, 1947 14Sheets-Sheet 13? :Clz r (H382) 380/ `c|3e|32 SUB OFFIQE 40 |380 p-l Cl 'rp. TRUNK om. |26'o l R|38O |325 y 0.0. RTZO BAT Dec. 25, 1951 P. BAKKER 2,579,721

' COMMUNITY AUTMATIC EXCHANGE NETwoRK ATT Patented Dec. 25, 1951 UNITED STATES PATENT OFFICE COMMUNITY AUTOMATIC EXCHANGE NETWORK Pier Bakker, Chicago, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, lll., a corporation of Delaware 7 Claims.

The present invention relates to telephone systems and more particularly to community automatic exchange (C. A. X) networks. This application is a division of the copending application of Pier Bakker, Seri al No. 762,715, filed July 22, 1947.

In a C. A. X network, there is frequently provided a toll oiice including a switchboard, a tandem oflice of the automatic type, and a sub-oflice of the automatic type; whereby traine between the switchboard in the toll oiiice `and the subscriber substations in the sub-office must be trunked through the tandem cnice. Also the tandem oiiice frequently comprises restricted service features preventing some of the subscriber substations therein from completing connections directly to subscriber substations in the sub-office, without the aid of the operator at the switchboard in the toll office.

It is a general object of the present invention to provide in a C. A. X network of the character described, improved switching apparatus for setting up the various connections between the different oflices in a simple and efficient manner without sacricing supervision or complicating the directory number plan.

Another object of the invention is to provide in a telephone system of the type noted, an improved trunk selector-digit sender and repeatertrunk selector arrangement in which iirst automatic switching apparatus is responsive to a given digit to select either the repeater-trunk selector or the trunk selector-digit sender depending upon trafc conditions; whereupon the lselected repeater-trunk selector selects lan idle `trunk line extending to the desired destination,

or the selected trunk selector-digit sender selects an idle trunk extending to second automatic switching apparatus and then operates the second switching apparatus to select an idle trunk line extending to the desired destin-ation.

Another object of the invention is to provide a trunk selector-digit sender of improved connection and arrangement.

Another object of the invention is to provide a repeater-trunk selector of improved connection and arrangement.

Further features of the invention pertain to the particular arrangement of the circuit elements of the system, whereby the above-outlined and additional operating features thereof are attained.

Sect. 1.--The general .arrangement of the telephone system The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings in which Figure l is a schematic diagram oi the area served by a telephone system embodying the present invention and illustrating the general arrangement of the apparatus incorporated in several of the oiiices thereof; and Figs. 2 to 14, inclusive, taken together, illustrate the details of certain of the apparatus incorporated in the telephone system, which apparatus has embodied therein the features of the invention as briey outlined above. Specifically, in order to form a unied diagram, Figs. 2 to 9, inclusive, should be arranged vertically in sideby-side relation from left to right, and Figs. 10 to 14, inclusive, should be arranged vertically in side-by-side relation from left to right and disposed respectively below Figs. 5 to 9, inclusive.

More particularly, Fig. 2 illustrates the details of one of the toll line circuits incorporated in the toll cnice; Figs. 2 to 4, inclusive, illustrate the details of one of the toll circuits incorporated in the tandem omce; Figs. 5 and 6 illustrate the details of the repeater and trunk selector incorporated in the tandem office; Figs. and l1 illustrate the details of one of the trunk circuits incorporated in the tandem office; Figs. 7 to 9, inclusive, illustrate the details of a trunk circuit incorporated in the sub-cnice; Figs. 12 and -13 illustrate the details of one of a group of trunk circuits incorporated in the sub-cnice; and Fig. 14 illustrates the details of a trunk selector and digit sender incorporated in the suboflce.

Referring now more particularly to Fig. l, the telephone system there illustrated serves an area comprising a toll oiiice I0, a magneto ozice 20, a tandem oiiice 3l) and a sub-office 40, the toll cnice lll being disposed relatively remote from the tandem office 353, and the tandem oiiice 30 being disposed relatively near to the magneto oilice and to the sub-office 40. The toll oiiice Ill serves subscriber substations of lany suitable type and comprises a manual dial switchboard 200; while the magneto cnice 20 serves subscriber substations of the magneto type and comprises a manual dial switchboard 286. The tandem office is a relatively small office of the automatic type, serving a number of different groups of subscriber substations rendered different classes of service and at least one group of paystations. One of the groups of subscriber substations includes the subscriber substation Tl. Also, the sub-oilice is a relatively small olice of the automatic type serving a group of subscriber substations, including the subscriber substation T2.

The toll oice Il] and the tandem oiilce 30 are interconnected by a small group of toll lines; which group may comprise three individual toll lines, including the toll line 220. The magneto office 2t and the tandem oiiice 30 areinterconnected by a small group of trunk lines; which group may comprise three individual trunk lines, including the trunk line itil. The tandem cnice 30 and the sub-office lill are interconnected by a single trunk line Silit; and also by a small group of trunks; which group may comprise two in" dividual trunks, including the trunk H961. Each of the toll lines 225i, etc., terminates in toll line circuits respectively disposed in the toll office lil and in the tandem oilice 3d, the toll line 220 terminating in the toll line circuits 21e and 250 respectively disposed in the toll office l@ and in the tandem office 36. Each of the trunk lines 23S, etc., terminates in trunk line circuits respectively disposed in the magneto office 2@ and in the tandem oice 3Q, the trunk line 286 terminating in the'trunk line circuits 285 and 28! respectively disposed in the magneto olice 2i@ and in the tandem oilce 3i?. The trunk line 59d terminates in a repeater Yand trunk selector 50@ disposed in the tandemomce 3i! and in a trunk line circuit 100 disposed in the sub-oiice 4t. Finally, each of the trunks l itil, etc., terminates in trunk circuits respectively disposed in the tandem ofice 3i! and in the sub-Omoo Alt, vthe trunk Il'terminating in the trunk circuits Hitt and lil respectively disposed in the tandem oiiice 3@ and in the sub-cnice 40. In the toll ofce l0, each of the toll line circuits ZI, etc., is accessible to the switchboard 260;, and in the magneto oihce 2i), each of the trunk line circuits 2&5, etc., is accessible to the switchboard 2&6.

The tandem oice 3S also comprises automatic switching apparatus including a plurality of nder-connector links 3%0, etc., provided with an associated distributor 338, and two incoming connectors 39?, etc., respectively individually associated with the trunk circuits ltl etc., the trunk circuit mil-ll being connected to the individually associated incoming connector 3G@ by a trunk lll. Each toll line circuit Bilt, etc., is provided with a line circuit individual thereto, the toll line circuit 25% being provided with )the line circuit 384 individually connected thereto by the trunk 33t. Also each trunk line circuit 281, etc., is provided with a line circuit individual thereto, the trunk line circuit 281 being provided with the line circuit 335 individually connected thereto by the trunk 233. Fnally, each subscriber substation is provided with a line circuit individual thereto connected to the associated vsubscriber line, the

and each trunk line circuit Zl, etc., is provided with an incoming trunk, the trunk line circuit 261 being provided with the incoming trunk 39 Also the repeater Yand trunk selector 5t@ is provided with an incoming trunk ddii; and each of the trunk circuits ldiidetc., is provided with an incoming trunk, the trunk circuit istil being provided with the incoming trunk 45de. The incoming' trunks 6%, etc., extending to the toll line circuits 258, etc., are accessible to the repeater and trunk selector 56e and to the two incoming connectors 3&6, etc.; while the trunks li'iii, etc., eX- tending to the toll line circuits 25%, etc., are aocessible to the connectors of the various links Seil, etc. The incoming trunks till, etc., extending to :the trunk line circuits 26 i, etc. are accessible both to the connectors of the links 39%, etc.,and to the two incoming connectors Stili, etc. The various subscriber lines 3&6, etc., are accessible to the connectors of the various links Bt, etc., and to the two incoming connectors 360, etc. The incoming trunk 49@ extending to the repeater and trunk selector ilii is accessible to the connectors of the Various links 33t, etc.; while the incoming Atrunks 4%, etc., extending to the trunk circuits |039, etc. are accessible'to the connectors of the various links 39e, etc.

The sub-cnice it comprises automatic switching apparatus, including a plurality of iinderconnector links 880i, etcz, provided with an associated distributor 883; andY an incoming connector individually associated with the trunk line circuit wie and connected thereto by the trunk Edil. Each trunk circuit i239, etc., isl provided with a line circuit individual thereto, the trunk circuit EN@ being provided with theline circuit 'iiil individually connected thereto by the trunk lge. vided with a line circuit individual thereto connected to the associated subscriber line, the' subscriber substation T2 being provided with the line circuit 832 individually connected to the associated subscriber line 38|. The various line circuitsfl'i, 882, etc., are respectively connected to theY distributor 883 and are accessible to the nders of the various links 884, etc. Each trunk circuit i263, etc., is provided with an incoming trunk, the trunk circuit H2M being provided with the incoming trunk S3363; and the trunk line circuit 's't is provided with an incoming trunk r91%. Also the sub-cnice it comprises a trunk selector and digit sender lfll provided with an incoming trunk 3%. The incoming trunks 133e, Vgli, ESQ, etc., are accessible to the connectors of the various links 3M, etc.; while the trunk selector and digit sender Het has access to the incoming trunks lt, etc., extending to the trunk circuits i290, etc. Finally, both the connectors of the various links li, etc., and the incoming connector 835 have access to the various subscriber lines 238i, etc.

Sect. 2.-Thc apparatus incorporated in the telephone system In the toll office Iii, the switchboard is of the conventional manual dial type, includingcord circuits, not shown. Each of the toll line circuits is identical to the toll line circuit 2m; which comprises, as shownV in Fig. 2, a relay group including a simplex pulse relay RZI and a signal relay R2i5, as well as a repeating vcoil RCZI'I.

In the magneto oce 20, the switchboard 235 is of the conventional manual dial type and may be identical to the switchboard 20. Similarly, the trunk line circuits 235, etc., may be identical to the toll line circuit 2 It.

In the tandem oice, the toll line circuits 25S, etc, and the trunk line circuits 23H, etc., may be identical; the toll line circuit 25S comprises, as Vshown in Figs. 2, 3 and 4, a repeating coil RC223, a timer 25, and a relay group including a signal relay REN, a supervisory relay R250, a dial relay R256, a pulse relay RZil, an answer relay R279, a switch relay R399, a control relay R320, a line relay R338, a time release relay R3liii, two hold relays R350 and R359, a time release relay Rtl, a line relay Rfii, a timer relay Rfil, a seize relay Rii, a tone relay Rd, two start relays R445 .and R450 and a reverse battery relay R560.-

The repeater and trunk selector 59B comprises,

as shown in Figs. 5 and 6, a repeating coil RCBSD Each subscriber substation is pro-l Y anda relay group including three select relays R5|0, R520 and R530, a reverse battery relay R540, a busy relay R550, a test relay R560, a release relay R510, a control relay R580, a line relay R590, a hold relay R6|0, a dial relay R620, a cutoff relay R630, an answer relay R640, a supervisory relay R650, a start select relay R655, a simplex relay R668, a ring-down relay R665 and two start relays R610 and R680.

The trunk circuits |000, etc., are identical; and theV trunk .circuit |000 comprises, as shown in Figs. and 11, a repeating coil RCI |15, a timer, not shown, and a relay group including a reverse battery relay RI 0|0, a cut-off relay RI 020, a timer relay R|030, a seize relay R|040, a control relay R|050, a dial relay R|060, a hold relay R|010, two line relays R|080 and RII |0, a supervisory relay RI |20, a time release relay RI |30, an answer relay RI |40, a switch relay R| |58, a hold relay RI |60, and a simplex relay R| |10.

Also, in the tandem ofiicc 30, the line circuits 384, 385 and 381 may be of any suitable types, while the distributor 388 is of any conventional type. The subscriber substations TI, etc., are provided with conventional substation equipment; and the paystations, not shown, are of the conventional prepayment type. Further, the private subscriber lines are provided with call registers and meters, not shown. Finally, the lnder-connector links 390, etc., are of the Strowger restricted service type disclosed in the co-pending application of Pier Bakker, Serial No. 723,525, led January 22, 1947 while each of the incoming connectors 300. etc., is of the Strowger type and is substantially identical to the connector portion of the link 390. In passing, itis noted that the link390 is of the mixed service type and is responsive to the single digits '1, 8, 9 and 0, respectively, to select the groups of trunks terminated in the rst, second, third and fourth levels thereof and then automatically to operate in order to select an idle trunk in the previously selected group or level. Also the link'390 is responsive to the prefix digits 2 or 3 followed by one of the digits 5, 6, '1, 8 and 9 to select the respectively' corresponding fth, sixth, seventh. eighth and ninth levels thereof, terminating the lines extending to the subscriber substations and paystations. Thereafter, the link 380 is responsive to any third digit from 1 to 0 to select the respectively corresponding 'line in the previously selected level. Finally, the finder-connector link 390 is responsive to any fourth digit from 1 to 0 to Ybring about code ringing of the selected line in the event it is idle. The combination of the Iirst or prefix digit and the fourth code digit permits the link 380 to ring the called line with any one of twenty individual codes; whereby as many as` twenty party subscriber substations may be arranged or any single party line.

Also the link 390 comprises a restricted service arrangement, whereby certain of the subscriber lines may not extend connections via the two groups of trunks that are respectively selected in response to dialing the digits '1 and 9; and none of the subscriber lines may extend connections via the trunks that are selected in response to dialing the digit 8.

In the sub-olice 40, the trunk line circuit 105 comprises, as shown in Figs. '1, 8 and 9, a repeating coil RC160, a timer 936 and a relay group including two ring-down relays R1|0 and R120, a switch relay R130, a line relay R140, an answer relay R150, a hold relay R8|0, a simplex relay R820, a hold relay R830, a time release relay 'the' various oices.

R840, a dial relay R850, a controlrelay R860, a line relay R810, a tone relay R8|0, a seize relay R920, two start relays R930 and R940, a timer relay R850 and a reverse battery relay R060. t

Also, the trunk circuits |200, etc., are identical to the trunk circuit |000; the trunk circuit |280 comprising, as shown in Figs. 12 and 13, a repeating coil RC|215, a timer, not shown, and a relay group including a reverse battery relay R|3|0, a cut-oi relay R|320, a timer relay R|330, a seize relay R|340, a control relay R|350, a dial relay R|360, a hold relay R|310, two linerelays R|380 and R|2|0, a supervisory relay R|220, a time release relay R|230, an answer relay R|240, a, switch relay R|250, a hold relay R|268 and a simplex relay R|210. l

Also in the sub-office 40, the line circuits 110, 882, etc., may be of any suitable types; while the distributor 883 is of any conventional type. The subscriber substations T2, etc., are provided with conventional substation equipment; while links 884, etc., and the incoming connector 885, are of the conventional Strowger types.

Finally, the trunk selector and digit sender |400 comprises, as sh-own in Fig. 14, a relay group including two select relays R|4|0 and R|420, a busy relay R|43ii, a test relay R|440, a cutoi relay RI450, three pulse relays R|485, R|455, and H4456, a shunt relay R|410, a line relay R|415, a control relay R|480, and a start relay R|496. Also, the trunk selector and digit sender |406 comprises a send switch |40| of the rotary type, including two wipers |402 and |403 provided with a magnet M|484 for driving the wipers noted step-by-step in the clockwise direction away from their home positions with respect to the associated contact banks.

A ybetter understanding of the connection and arrangement of the apparatus incorporated in the telephone system will be facilitated froma considerationof the extension of calls involving Sect. 3.-Local calls in the tandemomce First assuming that a call is initiated at the subscriberlsubstation TI in the tandem oflice 30 that is to be completed locally to another subscriber' substation therein, the line circuit 301 is controlled via the subscriber line 386 in order to govern operation of the distributor 388; whereby an idle one of the links, such, for example, as the link 380 is assigned thereby. The finder of the assigned link 390 operates' to find the sub? scriber line 386 extending to the calling subscriber substation T|; whereby the line circuit 381 is operated in order to mark the subscriber line 386 as busy to the other links 390, etc. Also the connector of the link 300 operates to return dial tone over the subscriber line 385 to the calling subscriber substation TI; whereby the subscriber at the calling subscriber substation T| proceeds to dial the directory number of the called subscriber substation, which number might be 3654. The first digit 3 received by the link 300 indicates thatthe present call is a local call and selects a portion of the ringing code. The *connector of the link 360 responds to the second 4digit 6 to select the corresponding sixth level; and then responds to the third digit 5 to select the corresponding fth line in the previously selected level; which line extends to the. called subscriber substation. The connector of the link 390 then responds to the fourth digit 4 to select the `remainder of the ringing code that is to be employed to ring the called subscriber line. The

7 first or prex digit 3, in conjunction with the fourth digit 4, selects one of twenty ringing codes at` this time; whereby the particular selected ringing code is 4projected over thercalled subscriber line in order to signal the corresponding one Vof the` twenty party subscriber substations connected to the called subscriber line, assuming the called subscriber line is of the twenty party typeand provided it is idle at this time. The operation of the connector of the link 390 to return busy tone current to the `calling subscriber substation TI in the event the called subscriber line is busy is in accordance with conventional practice; and the remainder of the operation of the connector of the link 390 in response to the answering of the call on the called subscriber line, as well as `the subsequent release of 'the established connection, are in accordance with conventional practice.

In view of the foregoing, it will be understood that' a call from a subscriber substation in the tandemVv exchange 30 may be completed locally to a playstation therein in an identical manner. Also, it will be understood that a call may be initiated at a paystation in the tandem exchange 30 by depositing a coin or check thereat and then subsequently completedr locally to a called subscriber substation or p-aystation therein ina conventional manner.

' Sect. 4.--A call from a subscriber substation or a pag/Station, in' the tandem oce to a subscriber substation in. the sub-oce First assuming that a'call is to be extended from a'subscriber substation, such, for example, as the subscriber substation TI in the tandem oice 30 to a called subscriber substation, such, for example, as the subscriber substation T2 in thesub-oice 4|),v the subscriber at the calling subscriber substation TI first initiates the call; whereby the link 390 may be taken for use in the manner previously explained. The subscriber at the calling subscriber substation T| then dials the prefix digit 9 followed by the directory number of the called subscriber substation T2; which number might be 23; whereby 9-23 may be dialed. The connector of the link 390 responds to the digit 9 to select the correspondingl third level and then an idle trunk terminated therein, such, forexample, as the trunk 400 extending to the trunk circuit |000, the trunk400 being marked as' idle to the connector of the link 390 by the absence of ground potential upon the control conductor C403 thereof. When the connector of the link 390 seizes the trunk 400, a loop circuit is completed for energizing in series the windings of the line relays RI080 and RI l in the trunk circuit |000; this loop circuit extending from ground potential by way of the winding of the ,line relay R|080, the contacts H54, the lower left-hand winding of the repeating coil RCI |15 and the contacts |0I4 to the line conductor C402 of thetrunk 400, and from battery potential by way of the winding of the line relay RI I I0, the contacts ||52 the upper left-hand winding of the repeating coil RCI |15 and the contacts |0|2 to the line conductor C40| of the trunk 400. At this time the line conductors C40| and C402 of the trunk 400 are connected together by way of the link 390 and the subscriber line 386 at the calling ,subscriber substation TI. When this loop circuit is thus completed, `both of the line relays R|080 and RI I0 operate. Upon operating, the line relay RI I0 completes, at the contacts acircuit for operating the hold relay R|010.

Upon operating, the hold relay R'I01'0 completes l at the contacts |013, a path,'including the contacts I3, for applying battery potential by way of the winding of the supervisory relay R| |20 to the simplex conductor CI |16; which simplex con-v ductor CI |16 is connected by way of the upper and lower right-hand windings ofthe repeating coil RCI |15, the line conductors of the trunk and the upper and lower left-hand windings of the repeating coil RCI215 to the simplex conductor CI215; which simplex conductor C|2116 .is connected by way of the contacts |314 and |353 and the winding of the simplex relay R|210 in the trunk circuit |200 `to ground potential. Accordingly, at this time the winding of the'supervisory relay Rl |20 in the trunk circuit |000 is energized in series with the winding of the simplex relay R|210 in the trunk circuit |200; whereby both the supervisory relay R||20 and the simplex relay RI210 operate. Also upon operating, the hold relayR|010 in the trunk circuit Iillloompletes, at the contacts |012, a circuit, including the contacts |034, for operating the time ,release relay RI |30. Upon operating, the' time release relay RI |30 completes, at the contacts H34, a path for applying ground potential to the control conductor C403 of the trunk 400, thereby to mark the trunk 400 as busy to the other links 390 etc., having access thereto. Also the time release relay R||30 completes, at the contacts |35, a path for applying ground potential to the timer start conductor CI |38 in order to initiate operation of the associated timer, not shown. li'urtherrtheV time release relay R||30 completes, at the contacts I |31, a path, including the contacts ||i2| and |032, for applying battery potential by way of the winding of the timer relay R|030 to the timer conductor C|024;V however, the timer relay R|030 is not operated at this time.

Upon operating the simplex relay .RI210 in the trunk circuit |200 completes, at the contacts |213, a circuit for operating the hold relay R|260. Upon operating, the hold relay R|260 completes, at the contacts |262, a circuit for operating the switch relay R|250 and a multiple circuit for energizing the lower winding of the answerrelay R|240. When the lower winding of the answer relay R|240 is thus energized, it does not operate at this time. Upon operating the switch relay RI250 interrupts, at the contacts |252 and |254, Y

normal connections between Vthe windings of the line relays l'\.|2||l and R|380 and the upper righthand winding and the lower right-hand winding, respectively, of the repeating coil RCI215; and completes, at the contacts |25I, |253, |255'and |253, a path, including the resistor |224 and the upper right-hand winding and the lower righthand winding of the repeating coil RCI215, for bridging the upper wind-ing of the answer relay RI240 across the line conductors C|28| and C|282 of the trunk |280 extending to the line circuit 110. Further, the switch relay R|250 completes, at the contacts |259, a path, including the contacts |233, for applying ground potential. to the control Vconductor CI 383 of the trunk |380, thereby to mark the trunk |380 as busy to the `various links 084, etc., having access thereto.

When the upper winding of the answer relay R|240 is thus bridged across the line conductors C|28| and C|202 of the trunk |280, it is energized, but it is not poled to operate. Also, the line circuit operates to control the distributor 883, whereby an idle one of the finder-connector links, such, for example, as the link 834 is assigned for `use at this time. The `finder of the link 884 nds the calling trunl; |285 marked by the line circuit `170; whereby the connector of the link 858i is conditioned to be responsive to digits received over the trunk |280.

When the subscriber at the calling subscriber trunk circuit Iilii follow. Each time the line relay RI |53 restores and then reoperates, it interrupts and then recompletes, at the contacts l I i I, the previously mentioned circuit or maintaining operated the hold relay Rl li'ii whereby the latter relay remains operated during impulsing as it is of the slow-to-release type. Also each time the line relay RI I restores and then reoperates, it completesand then interrupts, at .the contacts |||2, a circuit including the contacts ll and I |32, for operating the dai relay RItt; whereby the latter relay operates and remains operated during impulsing as it is of the slow-to-release type. Upon operating, the dial relay Rlilll completes, at the contacts |653, a path including the contacts lil'il, for connecting the resistor |925 in vshunt to the winding of the line relay BMSG in order to improve iinpulsing over the loop `circuit extending back to the calling subscriber substation TI. Also the dial `relay Rmb@ completes, at

thecontacts |665, a path, including the contacts f M35, for connecting the resistor H in shunt to the winding of the supervisory relay RI |20 in order to impro-ve impulsing over the simplex conductor C! Ili. Also each time the line relay RI I |t restores and then reoperates, it interrupts and then recornpletes, at the contacts IHS, the

previously traced circuit for energizing in series the winding of the supervisory relay RI|2i in the trunk; circuit met and the winding of the simplex relay Rllllil in the trunk. circuit llil; whereby the relays mentioned follow; following by the `supervisory relay RI I2@ in the trunk circuit Iil being without effect.

l The rst time the simplex relay RIZ'ii) in the trunk circuit |299 restores and then reoperates, it completes and then interrupts, at the contacts |.2'i2, a circuit, including the contacts Het, |263 and |343, for energizing in seriesthe upper and lower windings of the seize relay RI 34E; whereby the latter relay operates. `Upon operating the seiaerelay Rllil completes, at the contacts Idil?,

a holding circuit, including the contacts Is and the grounded control conductors CI 253 of the trunk .I 289, for energizing in series the upper and lower windings thereof. Also each time the sirnplex relay RIZ'lr'i restores vand then reoperates, it

, interrupts and then recornpletes, at the contacts path, including the resistor |223 and the contacts |'26|, 121| and |25I, across the vline conductors lC|2Il| and C|2il2 of the trunk |239; 'thereby to :improve impulsing over the trunk litt and the line circuit 710 to the connector ofthe linl: 884. Finally, each time `the simplex relay R|2lc re- 10 stores and then reoperates, it interrupts and then recompietes, at the contacts IZlI, the previously traced path across the line conductors C I 23| and (N282 of the trunk |2253; whereby the impulses of the second digit `2 are transmitted over the trunk li and the line circuit to the connector of the link 884 in order to `eilect corresponding operation thereof.

At .the conclusion of the second digit 2 dialed at the calling subscriber `substation Ti, the line relays 11i-ISBD and RII It in the trunk circuit I't remain operated; whereby the hold relay Rlill is retained in its operated position and shortly thereafter the dial relay Riiitii restores. Upon restoring the dial relay Rlll removes: the resistors M225 and |25 from across the winding of the line relay and the winding of the supervisory relay Rlll, respectively. Also at this time the winding of the supervisory relay RI 22B in the trunk circuit lodi) and the winding of the simplex relay Rlrhl in the trunk circuit |23@ are steadily energized in series relation; whereby the relays mentioned are retained in their operated positions.

The operated simplex relay RIZi in the trunk circuit I 2te retains operated the hold relay RI 26.3 and effects restoration of the dial relay Riii shortly thereafter, since the latter relay is of the slow-to-release type. Upon restoring the dial relay Rlsii interrupts, at the contacts |36I, the previously traced path for connecting the resistor |223 across the line conductors C|2S| and C|282 of the trunk lftil; whereby the upper windingof `the answer relay RIfZlil is no longer short-cir cuited by the resistor |223. Also upon restoring, the dial relay Rl3il completes, at the contacts ISSii, a holding circuit, including the contacts iilftl, istil, |257 and |252, for energizing in series the upper and lower windings of the seize relay RIBAB; and a multiple path, also including the contacts |258, for applying .ground potential in the trunk circuit |26@ to the control conductor C|283 of the .trunk |286, thereby to retainoperated the link 884.

In `View of the foregoing explanation of the manner in which the second digit 2 dialed at the callingsubscriber Isubstation TI is received by `the trunk circuit Illllil and repeated therefrom over the trunk |59!) vto the trunk circuit |200 and then repeated therefrom to the connector of :the link 884, it will be .understood that the third digit 3 -dialed at `the calling subscriber substation TI is transmitted in an identical manner to the connector of the link `884. The connector of the link 88d `responds to the digits 2 `and 3 to select the subscriber line 228| extending -to the called .subscriber substation T2. The subsequent operation of the connector of the link 884 `depends upon the idle or busy condition of the called subscriber substation T2 at this time; and in the event the called subscriber substation T2 is busy at this time, the connector of the link 88d operates-lin order to return busy 'tone current over the .trunk |28I) to the trunk circuit |2li0; whichbusy tone current `traversing the upper right-hand winding and the lower right-hand winding ofthe repeat- Ling coil RC|27|5 induces a corresponding busy tone current in the left-hand windings thereof. `The `'busy tone current is returned over the `trunk H90 :and traversing the right-hand windings of the repeating coil RCI 5 `induces a corresponding busy tone vcurrent Ain the upper left-,hand winding and `thelower left-hand winding thereof; which busy tone current yis, returned over theloop circuit to the .calling subscriber substation TI in order to indicate to .the subscriber thereat that the desired connection cannot be had at this time by virtue of the busy condition of the called subscriber substation T2.

On the other hand, in the event the called subscriber substation T2 is idle at this time, ringing current is projected from the connector of the link 884 over the subscriber lineBSI thereto and the line circuit 882 is operated in order to mark the subscriber l-ine 88| as busy to the other 3 links 884, etc., having access thereto. Also, ringback tone current is returned from the connector of the link 884 over the previously traced connection to the calling subscriber substation Tl in order to indicate to the subscriber thereat that the called subscriber substation T2 is being rung. When the subscriber at the called subscriber substation T2 -answers the call, the connector of the link 884 operates in order to reverse the polarity of the trunk |280; whereby the upper winding of the answer relay Ri240 is poled to operate. Upon operating, the answer relay` R|240 completes, at the contacts |242, a circuit for energizing the upper winding of the control relay R|350; whereby the latter relay operates. Upon operating the control relay R|350 completes, at the contacts |3511` a holding circuit, including the contacts |222 and |250 and the resistor |225, for energizing the winding of the simplex relay Rl 210; interrupts, at the contacts |353, the previously traced original path for connecting ground potential by way of the winding of the simplex relay R|2`|0 to the simplex conductor C|216 and completes, at the contacts |352, a path, including the'contacts |314 for connecting battery potential by way of the winding of the supervisory relay R|220 to the simplex conductor C|2'l6. Accordingly, at this time the Winding of the supervisory relay R|220 in the trunk circuit |200 is shortcircuited in series withv the winding of the supervisory relay R||20 in the trunk circuit |000;

whereby the supervisory relay R|220 is retained in'its restored position and the supervisory relay R| |20 restores. Y

Upon restoring, the supervisory-relay R| |20 in the trunk circuit |000 completes, at the contacts VI |2l, a circuit, including the contacts |0'||, |062 "and ||3|, for energizing the winding of the cutoi relay R|020, and a multiple circuit, also including the contacts |08|, for energizing the fwinding of the reverse battery relay R||0; *whereby the cut-oir relay R|020 and the reverse battery relay R|0|0 operate shortly thereafter,

both of these relays being of the slow-to-operate sociated timer, not shown. Upon operating, the L reverse battery relay RI 0 0 completes, at the contacts |0|5, a holding circuit for energizing the Vwinding thereof, and including the contacts |3I,

||2|, |062 and |0'||. Also the reverse battery Vrelay Rf|0|0 reverses, at the contacts |0| |0|2, 4|0|3 and |0|4, the polarity of the trunk 400 with respect to the line relays R|080 and RI I0; which reversal of polarity over the trunk 400 and the Y Vlink 390 is effective to operate a meter, not shown,

individually associated with the calling subscriber substation TI or otherwise to assess a charge against the calling subscriber substation Tl. At this time an established connection is completed between Vthe calling subscriber substation Tl'in.

12 the tandem oce 30 :and the called subscriber substation T2 in the sub-ofce 40,. 1 At the conclusion of the established 'connection, when the subscriber at the called subscriber substation T2 disconnects, the connnector of the link 884 is operated in order again to reverse the polarity of the trunk |280; whereby the'answer relay RI240 in the trunk circuit |200 is poled to restore. Upon restoring, the answer relay RI240 interrupts, at the contacts |242, the previously mentioned circuit for energizing the upper Winding 'of the control relay R|350, thereby'to cause the latter relay to restore. Upon restoring, the control relay R|350 interrupts, at the contacts |352, the previously traced path for connecting battery potential by way of the winding of the supervisory'relay R|220 to the simplex conductor C|2'|6; interrupts, at the contacts |354, the previously traced holding circuit for maintaining operated the simplex relay R|210 and recompletes, at the contacts |353, the previously traced original path for applying ground potential by way of the winding of the simplex relay R|2`i0 to the simplex conductor CI216. VAccordingly, at this time, the winding of the simplex relay R|210 in the trunk circuit i200 is again energized in series with the winding of the supervisory relay R||20 in the trunk circuit |000; whereby the simplex relay R|210 is retained in its operated position and the supervisory relay Rl |28 reoperates. Upon reoperating, the supervisory relay Rl |20 interrupts, at the contacts ||2 the previously Ytraced circuits for maintaining operated the cut-ofi relay R|020 and the reverse battery relay RI 0|0. Upon restoring, the reverse battery relay R|0|0 again reverses, at the contacts |0||, |0|2, |0|3 and |0|4, the polarity of the trunk 400 with respect to the line relays R|080` and Rl l0 in the trunk circuit |000; however without eiect at this time. Upon restoring,

the cut-off relay R|020` again completes, at the Y taining operated the hold relay R|010; whereby the latter relay restores shortly thereafter, it being of the slow-to-release type. Upon restoring the hold relay R|8i`|0 interrupts, at the contacts |072, the previously traced circuit for maintaining operated the time release relay RI |30. Upon restoring', the time release relay Rl |30 'interrupts, at the contacts ||34,v the previously-men tionedpath for applying ground potential to the control conductor C403 of the trunk 400, thereby to effect the release of the link 390, and to mark the trunk 400 as idle to the links 390, etc.. having access thereto. When the link 390 isl thus released, the line circuit381 is operated in order again to mark the subscriber line 386 as idle to the links 390, etc.,` having access thereto. Also upon restoring, the time release relay RI |30 interrupts, at the contacts H35, the previously mentioned circuit for applying groundpotential to the timer start conductor CI |38` in order to arrest operation of the timer, not shown. Also, the time release relay RI |30 interrupts, at the 

